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United States Patent |
6,016,206
|
Koide
,   et al.
|
January 18, 2000
|
Image processing method and apparatus and image forming method and
apparatus using the same
Abstract
An image can be outputted with a high sharpness without deteriorating a
resolution. In order to output an image of a high picture quality while
suppressing the occurrence of a moire, an exposure amount is modulated in
accordance with pixel density information of the image divided into pixels
of a predetermined size by an exposure amount modulating unit in a light
scanning unit, thereby expressing an image dark/light state. In this case,
in a highlight density region in which a pixel density is equal to or less
than 1/3 of the maximum image density, the density data of two adjoining
pixels is modulated by one pixel and the other pixel is not recorded. In a
density region in which the image density lies within a range from 1/3 to
1/2 of the maximum image density, a part of the density data of one of the
two adjoining pixels is transposed to the other pixel. In a density region
in which the image density is equal to or larger than 1/2 of the maximum
density, the pixel transposition is not performed.
Inventors:
|
Koide; Jun (Tokyo, JP);
Sasanuma; Nobuatsu (Yokohama, JP);
Ikeda; Yuichi (Tokyo, JP)
|
Assignee:
|
Canon Kabushiki Kaisha (Tokyo, JP)
|
Appl. No.:
|
575141 |
Filed:
|
December 19, 1995 |
Foreign Application Priority Data
| Dec 20, 1994[JP] | 6-316704 |
| Jul 12, 1995[JP] | 7-175989 |
| Dec 15, 1995[JP] | 7-327334 |
Current U.S. Class: |
358/3.02; 347/131 |
Intern'l Class: |
H04N 001/38 |
Field of Search: |
358/298,455-461,465,466,406
347/131,133
|
References Cited
U.S. Patent Documents
4561025 | Dec., 1985 | Tsuzuki | 358/298.
|
5124802 | Jun., 1992 | Ito et al. | 358/298.
|
5130808 | Jul., 1992 | Kemmochi | 358/298.
|
5148287 | Sep., 1992 | Kenmochi et al. | 358/298.
|
5148387 | Sep., 1992 | Kemmochi et al. | 358/298.
|
5359433 | Oct., 1994 | Nagase et al. | 358/466.
|
5418618 | May., 1995 | Kagawa et al. | 358/298.
|
5450212 | Sep., 1995 | Asada | 358/298.
|
5574833 | Nov., 1996 | Yoshiaki | 358/456.
|
Foreign Patent Documents |
0398763 | Nov., 1990 | EP.
| |
Primary Examiner: Riley; Shawn
Assistant Examiner: Toatley, Jr.; Gregory James
Attorney, Agent or Firm: Fitzpatrick Cella Harper & Scinto
Claims
What is claimed is:
1. An image processing apparatus comprising:
input means for inputting image data represented by multi-density level
pixel data;
detecting means for detecting density levels of each pixel;
transposing means for transposing a density of a first pixel to a density
of a second pixel neighboring the first pixel, wherein a transposition
ratio corresponding to the first pixel changing adaptively according to
the density levels of the first and second pixels detected by said
detecting means; and
output means for outputting the image data processed by said transposing
means.
2. An apparatus according to claim 1, further comprising:
binary coding means for converting the output image data into binary data;
and
image forming means for forming an image in accordance with the binary data
from said binary coding means.
3. An apparatus according to claim 2, wherein said binary coding means
comprises a pulse-width modulation circuit.
4. An apparatus according to claim 3, wherein said image forming means
comprises a laser beam printer.
5. An apparatus according to claim 3, wherein said image forming means
includes means for scanning on a photosensitive material using a light
beam modulated by pulse width modulating means.
6. An apparatus according to claim 5, further comprising developing means
for developing a latent image formed by the light beam on the
photosensitive material.
7. An apparatus according to claim 6, wherein said developing means
develops the latent image using a toner.
8. An apparatus according to claim 1, wherein the image data includes image
data of a plurality of color components.
9. An apparatus according to claim 8, wherein the plurality of color
components includes a black component.
10. An apparatus according to claim 9, wherein the transposition is
executed by said transposing means for at least the black component.
11. An apparatus according to claim 1, wherein said input means includes a
scanner for generating the image data.
12. An apparatus according to claim 11, wherein said scanner reads an image
by line scanning and outputs the image data, wherein the first and second
pixels are in a direction of the line scanning, and wherein a
transposition process of said transposing means is capable of differing
according to a line.
13. An image processing method comprising the steps of:
inputting image data represented by multi-density level pixel data;
detecting density levels of each pixel;
transposing a density of a first pixel to a density of a second pixel
neighboring the first pixel, wherein a transposition ratio corresponding
to the first pixel changing adaptively according to the density levels of
the first and second pixels detected in said detecting step; and
outputting the image data processed in said transposing step.
14. A printer for forming an image based on the image data output by said
image processing method of claim 13.
15. A printer according to claim 14, wherein said printer forms an image by
an electrophotograph method.
16. A printer according to claim 14, wherein said printer forms an image by
an ink-jet method.
17. A printer according to claim 14, wherein said printer forms a color
image.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to image processing method and apparatus and image
forming method and apparatus using such method and apparatus.
2. Related Background Art
Among conventional image forming apparatuses, for example, there is an
apparatus such that each pixel is expressed as a density by multi-values
in a dividing image of predetermined pixels, what is called a digital
image, by an electrophotography process. Exposure amounts of all pixels
are modulated in accordance with an image signal value of each pixel,
thereby reconstructing image information at a high fidelity. When the
exposure amounts of all pixels are modulated, however, there is a problem
such that a density gradation of an output image becomes nonlinear
(generally, not enough density is obtained in a highlight density region
and a gamma value is large in a halftone density region) for an input
image signal, so that a picture quality deteriorates.
As another example of the image forming apparatus, there is an apparatus
such that in the case of a pixel composition of an input image of a pixel
density of, for example, 400 d.p.i., an output image is set to 200 d.p.i.
in the main-scanning direction and to 400 l.p.i. in the sub-scanning
direction (direction perpendicular to the main scanning direction), and
the pixel density in the main-scanning direction (direction for light
scanning) is set to the half density and a density gradation of the output
image is made to linearly correspond to the input image, thereby forming
an image.
In the above conventional image forming apparatus, however, there are the
following problems. Namely, when the pixel density in the main-scanning
direction of the output image is reduced to 1/2 of the pixel density in
the sub-scanning direction, the whole resolution of the image is reduced
to 1/2 and sharpness of the image deteriorates. In case of outputting
image data of a high spatial frequency such as a mesh image or the like,
an interference with a recording pixel density easily occurs, and a moire
strongly appears.
To solve the above problems, in U.S. Pat. No. 5,148,287, U.S. Pat. No.
5,130,808, and the like, there is disclosed a technique such that when a
density level is low, by concentrating the dots, a density reproducibility
in a highlight density region is improved.
However, according to the method disclosed in the above two patents, there
are problems such that what is called a density jump occurs at a boundary
between the density region where the dots are concentrated and the density
region where the dots are not concentrated. Such a density jump is
observed as a pseudo outline by the human eyes, and the picture quality
deteriorates.
SUMMARY OF THE INVENTION
The invention is made in consideration of such problems and it is an object
of the invention to provide image processing method and apparatus and
image forming method and apparatus which can improve a gradation
reproducibility.
Another object of the invention is to provide image processing method and
apparatus and image forming method which can improve a gradation
reproducibility by a construction that is simpler than the conventional
one.
Still another object of the invention is to provide image processing method
and apparatus and image forming method which can prevent a generation of a
pseudo outline and can preferably form an image even in a highlight
density region.
Under the above objects, according to a preferred embodiment of the
invention, there is provided an image forming apparatus comprising: input
means for inputting digital image data every pixel; evaluating means for
evaluating a density of the image data inputted by the input means by at
least three stages; concentrating means for concentrating the digital
image data every pixel in accordance with the stage evaluated by the
evaluating means; modulating means for performing a signal modulation in
accordance with a level of the digital image data concentrated by the
concentrating means; and image forming means for forming an image in
accordance with the digital image data modulated by the modulating means.
Further, another object of the invention is to improve a gradation of an
image forming apparatus of an electrophotography system.
Further, another object of the invention is to provide a novel image
processing method for an image forming apparatus of the electrophotography
system.
The above and other objects and features of the present invention will
become apparent from the following detailed description and the appended
claims with reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic constructional diagram showing an embodiment of an
image forming apparatus according to the invention;
FIG. 2 is an explanatory diagram showing an image forming method of the
first embodiment according to the invention;
FIG. 3 is a characteristics graph showing each pixel modulating
characteristics of the invention;
FIG. 4 is an explanatory diagram showing an image forming method of the
second embodiment of the invention;
FIG. 5 is a characteristics graph showing each pixel modulating
characteristics of the invention;
FIG. 6 is a diagram showing a schematic construction of an image forming
apparatus of a form of the third embodiment of the invention;
FIG. 7 is a block diagram showing a main section of a scanner unit of the
form of the embodiment shown in FIG. 6;
FIG. 8 is a diagram showing a pixel modulation arrangement of the form of
the embodiment;
FIG. 9 is a diagram showing pixel modulating characteristics of the form of
the embodiment;
FIG. 10 is a diagram showing pixel modulating characteristics of the form
of the embodiment;
FIG. 11 is a flowchart showing the operation of a density processing unit
of the form of the embodiment;
FIG. 12 is a diagram showing an output result of another embodiment of the
invention;
FIG. 13 is a diagram showing density converting characteristics of the
embodiment shown in FIG. 12;
FIG. 14 is a flowchart showing a processing procedure to obtain an output
result shown in FIG. 12;
FIG. 15 is a cross sectional view showing a construction of still another
embodiment of the invention;
FIG. 16 is a diagram showing an output result of color dots formed
according to the apparatus shown in FIG. 15;
FIG. 17 is a diagram showing the relation between a toner grain size and an
RMS granularity;
FIG. 18 is a diagram showing visual sense MTF characteristics in least
distance of distinct vision; and
FIG. 19 is a diagram showing data of an apparatus which was experimented by
the present inventors.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Embodiments of the invention will now be described in detail hereinbelow
with reference to the drawings.
<Embodiment 1>
First, a schematic construction of a laser beam printer as an image forming
apparatus according to an embodiment of the invention with reference to
FIG. 1. The laser beam printer has a photosensitive drum 7 as a
photosensitive material which is supported in a printer main body 10 so as
to be rotatable in the direction shown by an arrow R2. A laser beam which
was modulated and oscillated by light scanning means (light scanning
apparatus) 1 is optically scanned to the photosensitive drum 7 through a
return mirror 2. Around the photosensitive drum 7, there are arranged:
charging means 3 for uniformly charging the surface of the photosensitive
drum 7 along a rotating direction of the photosensitive drum 7; developing
means 4 for forming a toner image by depositing a toner to an
electrostatic latent image formed on the photosensitive drum 7; transfer
charging means 5 for transferring the toner image on the photosensitive
drum 7 onto a transfer material; and cleaning means 6 for rotating the
toner remaining on the photosensitive drum 7 after the toner image was
transferred. Further, a sheet cassette 8 on which transfer material is
stacked is attached in the printer main body 10. A paper feed roller 9a
and a registration roller 9b to feed the transfer material in the sheet
cassette 8 to a position between the photosensitive drum 7 and the
transfer charging means 5 are provided in the printer main body 10.
A dot image forming block (not shown) for analyzing code data from a host
computer (not shown) and drawing a dot image into a memory is provided for
the light scanning apparatus 1. Exposure amount modulating means 15 for
modulating an exposure amount in accordance with image information and
expressing a light/dark state of an image is provided for the dot image
forming block. In case of a highlight density region as density data in
which an image density which is inputted is equal to or less than 1/3 of
the maximum image density, the exposure amount modulating means 15
modulates density data of two pixels by one (a) [pixel shown by "a" in
FIG. 2] of the two pixels and forms one dot and doesn't record the other
pixel (b) [pixel shown by "b" in FIG. 2]. In case of a region from a
halftone density region to a dark density region in which the image
density that is inputted is equal to or larger than 1/3 of the maximum
image density, the exposure amount modulating means 15 individually
modulates each pixel, thereby forming an image.
A method of forming an image by the laser beam printer of the embodiment
constructed as mentioned above will now be described.
First, the light scanning apparatus 1 converts the density data based on
the image information to an exposure amount of the laser beam through the
exposure amount modulating means 15. The laser beam which was modulated
and oscillated by the light scanning apparatus 1 optically scans the
surface of the photosensitive drum 7 which rotates in the direction of the
arrow R2 in the diagram through the return mirror 2, thereby forming a
potential latent image onto the photosensitive drum. In this instance,
however, it is assumed that the surface of the photosensitive drum 7 has
previously been charged by the charging means 3. A toner as a developing
agent is deposited onto the drum surface by the developing means 4 in
accordance with the surface potential of the photosensitive drum 7 on
which the latent image was potentially formed, thereby forming a visible
image. During this interval, a transfer material is pulled out from the
sheet cassette 8 of the transfer material by the paper feed roller 9a and
is conveyed by the registration roller 9b at a timing synchronized with
the rotation of the photosensitive drum 7. The transfer charging means 5
transfers the visible image formed on the surface of the photosensitive
drum 7 onto the transfer material at a timing matched with the timing to
convey the transfer material. The transfer material to which the visible
image was transferred is conveyed to fixing means 12 by conveying means
11. After the toner was fixed to the transfer material by the fixing means
12, the transfer material is ejected out onto a paper ejection tray 13
provided so as to be projected to the outside of the printer main body 10.
After completion of the transfer step, the toner remaining on the surface
of the photosensitive drum 7 is removed by the cleaning means 6. An image
is formed while repeating the series of processing steps as mentioned
above.
A method of modulating the exposure amount in accordance with the image
information and expressing a dark/light state of the image will now be
described with reference to FIGS. 2 and 3.
First, the image is decomposed to pixels arranged in the main-scanning
direction of a raster-shape as shown in FIG. 2. In case of a highlight
density region as density data in which image densities of two pixels a
and b which are adjoining in the main-scanning direction are, for example,
equal to or less than 1/3 of the maximum image density (in case of the
data in a range in the sub-scanning direction shown at A in FIG. 2), the
exposure amount modulating means 15 on the light scanning apparatus 1 in
the embodiment 1 adds the density data of two pixels, modulates the
addition result by one pixel a, forms one dot, and doesn't perform the
recording of the other pixel b. In case of a region from a halftone
density region to a dark density region as density data in which the image
density that is inputted is equal to or higher than 1/3 or the maximum
image density (in case of data in a range in the sub-scanning direction
shown at B in FIG. 2), the exposure amount modulating means 15
individually modulates each pixel and forms an image.
FIG. 3 shows the relation of the individual or synthesized output data
which is modulated for the synthesized input data of the two adjoining
pixels a and b. According to the relation, a dot which is formed by
modulating the density data of the two adjoining pixels by one of the two
pixels is dense and large. On the other hand, in case of forming an image
by modulating all of the pixels, the dot is thin and small and an exposure
amount which is generated for each pixel is small, so that the toner is
not deposited. Therefore, this embodiment reduces the inconvenience in
which the image is extinguished in the highlight density region or a
coarseness of the image occurs because a size of each dot that is formed
by the toner doesn't become stable and varies. A density reproducibility
and a density uniformity in the highlight density region are improved.
<Embodiment 2>
A method of modulating the exposure amount in accordance with the image
information on the basis of FIGS. 4 and 5 will now be described.
In case of the highlight density region in which the image densities of the
two adjoining pixels in the main-scanning direction are equal to or less
than 1/3 of the maximum image density as shown in FIG. 4, in a manner
similar to the embodiment 1, the density data of the two adjoining pixels
is modulated by one pixel c [pixel shown by "c" in FIG. 4] and two dots
are formed and the other pixel d [pixel shown by "d" in FIG. 4] is not
recorded. In case of a halftone density region in which the image
densities lie within a range from 1/3 to 2/3 of the maximum image density,
the exposure amount modulating means 15 which is used in the embodiment 2
sets the pixel c which was modulated in the highlight density region to a
fixed value as shown in FIG. 5 and modulates the density data in which the
density data of one pixel c is subtracted from the density data of the two
adjoining pixels by the other pixel d. Further, in a dark density region
in which the image densities are equal to or higher than 2/3 of the
maximum image density, each pixel is individually modulated and an image
is formed. Since the modulation of each pixel continuously changes for the
input density data as shown in FIG. 5, a pseudo outline is not generated
in the output image. Specifically speaking, for example, when forming a
gradation image whose gradation smoothly changes, in the foregoing
embodiment described by using FIG. 2, there is a possibility such that
pseudo outlines are generated in both of the density region shown at A and
the density region shown at B. However, according to the embodiment shown
in FIG. 4, such a possibility can be reduced. On the other hand, however,
since one modulation is performed by two adjoining pixels in a region from
the highlight density region to the halftone density region, namely, up to
the density region in which the image densities are less than 2/3 of the
maximum image density, resolutions in the region from the highlight
density region to the halftone density region deteriorate, a sharpness
deteriorates, and a generation intensity of a moire increases.
In case of forming an image by using the exposure amount modulating means
15 according to the embodiments 1 and 2, by using image data whose pixel
density is equal to or larger than 600 d.p.i., the pixel density in the
main-scanning direction in the highlight density region corresponds to 300
d.p.i. The dot arrangement of 300 d.p.i. cannot be distinguished by the
human eyes. There is, consequently, also an advantage such that by using
image data of 600 d.p.i. or higher, even if the method shown in the
embodiment is used, the deterioration in picture quality due to the
appearance of the image structure is not caused.
According to the embodiment as mentioned above, the density data of two
pixels is modulated by one of the two pixels and the other pixel is not
recorded, thereby reducing the pixel density in only the highlight density
region to the half in the main-scanning direction. Therefore, even if the
deterioration in resolution or the increase in moire intensity occur in
the highlight density region in which the image densities are equal to or
less than 1/3 of the maximum image density, since the density region is
the highlight density region, such phenomena are visually inconspicuous.
Thus, a sharpness as a whole output image doesn't deteriorate, the
generation intensity of the moire is not raised, the image in the
highlight density portion can be stably outputted, and an image of a high
picture quality can be outputted.
In the embodiment, in the highlight density region, the density data of two
adjoining pixels is modulated by the density data of one of the two pixels
and the other pixel is not recorded. However, it is also possible to
modulate the density data of a predetermined number of continuous pixels
by the density data of some of the pixels and not to record the remaining
pixels. With this construction, further, the resolution doesn't
deteriorate and an image of a higher sharpness can be obtained.
As described above, according to the image forming apparatus of the
embodiment, the light scanning means for forming the latent image onto the
photosensitive material is provided with the exposure amount modulating
means for modulating the exposure amount in accordance with the pixel
density information and expressing a dark/light state of the image for the
pixels divided into a predetermined size on the photosensitive material.
The exposure amount modulating means divides the image density region into
the highlight density region, halftone density region, or dark density
region, modulates the density data of a predetermined number of continuous
pixels in the light scanning direction by the density data of some of the
pixels in the highlight density region, and doesn't record the remaining
pixels. It is possible to prevent the formation of a dot which is so small
that the image formation cannot be stably performed. When the user tries
to form a dot such that the image formation cannot be stably performed,
dots are formed at random or dots are not formed. Therefore, an image
which the viewer feels has too much noise is produced. According to the
embodiment, consequently, a coarseness of the image due to a density
deterioration or a variation of the sizes of dots is improved.
In the region from the halftone density region to the dark density region,
by individually modulating each pixel, the resolution doesn't deteriorate
and an image with a high sharpness can be outputted. An image of the
highlight density portion can be stably outputted without raising the
generation intensity of the moire. An image of a high picture quality can
be outputted.
According to the embodiment 2 shown in FIG. 4, further, in the halftone
density region, one pixel modulated in the highlight density region is set
to the fixed value and as for the other pixel, the data in which the fixed
value is subtracted from the density data of two pixels is modulated, so
that a pseudo outline is not generated in the output image. In the dark
density region, by individually modulating each pixel, the resolution in
the dark density region doesn't deteriorate. Thus, the image of a high
sharpness can be outputted. The generation intensity of the moire is also
suppressed. The image of a high picture quality can be outputted.
FIG. 6 is a diagram showing a schematic construction of an image forming
apparatus of a form of the third embodiment of the invention. Although a
construction of the image forming apparatus is substantially the same as
that of the apparatus shown in FIG. 1, it will be explained hereinbelow.
Reference numeral 101 denotes a light scanning unit having a laser diode
and a laser diode oscillation control circuit (not shown). The light
scanning unit 101 oscillates in accordance with image data which is
transferred from a control unit 114 and forms an electrostatic latent
image onto a drum cylinder 107.
Reference numeral 102 denotes a return mirror for scanning a laser beam
from the light scanning unit 101 onto the drum cylinder 107.
Reference numeral 103 denotes a charging unit for charging a photosensitive
conductor material on the surface of the drum cylinder 107.
Reference numeral 104 denotes a developing unit for developing the
electrostatic latent image formed on the photosensitive conductor material
on the surface of the drum cylinder 107.
Reference numeral 105 denotes a transfer unit for transferring the formed
image which was developed by the developing unit 104 onto a transfer
material.
Reference numeral 106 denotes a cleaning unit for removing the remaining
toner or the like after completion of the transfer by the transfer unit
105.
Reference numeral 107 denotes the drum cylinder. A photosensitive conductor
material is coated onto the surface of the drum cylinder 107. The drum
cylinder 107 rotates in the direction shown by an arrow in the diagram.
Reference numeral 108 denotes a cassette to supply transfer material (not
shown); 109 a paper feed roller to take out the transfer material (not
shown) from the cassette; 110 a conveying roller for conveying the
transfer material (not shown); and 111 a conveying belt to convey the
transfer material.
Reference numeral 112 denotes a fixing unit for fixing the image formed by
transferring the developing agent onto the transfer material.
Reference numeral 113 denotes an ejection tray to hold the transfer
material ejected to the outside of the apparatus.
Reference numeral 114 denotes the control unit for performing a density
process, which will be explained hereinafter, to the image read data
inputted from a scanner unit 115 and transferring the processed data to
the light scanning unit 101.
Reference numeral 115 denotes the scanner unit for optically reading an
original image, converting the signal obtained to a digital signal, and
transferring to the control unit 114.
A schematic operation of the image forming apparatus having the above
construction will now be described. In accordance with an output timing of
the image data, the light scanning unit 101 operates in a manner such that
an oscillated light which is generated from a built--in laser diode (not
shown) is reflected by the return mirror 102. An electrostatic latent
image by the oscillated light onto the photosensitive conductor material
on the surface of the drum cylinder 107 which is rotated in the direction
of the arrow in the diagram and whose surface was charged by the charging
unit 103. In accordance with the surface potential of the photosensitive
conductor material on which the electrostatic latent image was formed, a
developing agent (for example, toner) is deposited by the developing unit
104, thereby forming a visible image and developing the electrostatic
latent image. The visible image deposited on the surface of the
photosensitive conductor material is transferred by the transfer unit 105
to the transfer material (not shown) which is conveyed by the conveying
roller 110 at a predetermined timing. The transfer material (not shown) to
which the visible image was transferred is conveyed by the conveying belt
111 to the fixing unit 112. The visible image transferred onto the
transfer material is semipermanently fixed by the fixing unit 112. After
that, the transfer material is ejected onto the ejection tray 113 on the
outside of the apparatus. The developing agent remaining on the drum
cylinder 107 is removed by the cleaning unit 106 by rotating the drum
cylinder 107.
In the form of the embodiment, although the image data which is transferred
to the control unit 114 is the image data read out from the scanner,
namely, although the apparatus shown in FIG. 6 is a copying apparatus, the
embodiment can be also easily applied to a printer which handles image
data that is transferred from a host computer.
A control construction of the scanner unit which is built in the apparatus
and executes image processes will now be described.
FIG. 7 is a block diagram showing a main section of the control unit 114 of
the form of the embodiment.
Reference numeral 121 denotes a CPU for performing controls of a reading of
image data into a line memory 122, a process of a density processing unit
123, a transfer of the processed image data to the light scanning unit,
and the like.
Reference numeral 122 denotes a line memory for storing the image data
inputted from the scanner unit 115.
Reference numeral 123 denotes the density processing unit for detecting
density data of the image data that is sequentially read out from the line
memory 122 and executing processes of the density data of the form of the
embodiment, which will be explained hereinlater. The density processed
data is outputted to the light scanning unit 101.
The density process of the image data in the form of the embodiment will
now be described with reference to FIGS. 8 to 11.
It is now assumed that the image forming apparatus which is used in the
form of the embodiment can express sixteen gradations.
In the density processing unit 123, in the case where the density data of
two adjoining pixels in the raster direction (main-scanning direction) is
a and b and the image density is equal to or less than the halftone
density, namely, in the case where the image density is smaller than C [C
denotes the maximum density of an image to be formed by the image forming
apparatus] (C>a+b), the density data of the pixel of density data b is
transposed to the pixel of density data a in accordance with arithmetic
operations of the following equations (1), thereby newly converting to the
pixels having density data of a' and b'.
All of the decimal portions of the values which are calculated by the
equations (1) are omitted.
a'=a+b{1-(a+b)/C}
b'=b{(a+b)/C) (1)
By performing the above process, the output result in each gradation of the
pixel of the density data a and the pixel of the density data b is as
shown in FIG. 8. In FIG. 8, the pixels which are adjoining in the
main-scanning direction have the same value. As will be also obviously
understood from the diagram, in the highlight portion (the 0th to 7th
gradations), the density data of the pixel of the density data b is
transposed to the pixel of the density data a in accordance with the
equations (1) and the resultant data is generated as output image data, so
that a difference occurs between the output results of the pixel of the
density data a and the pixel of the density data b. On the other hand, in
the dark portion (the 8th to 15th gradations), the output results of the
output image data of the pixel of the density data a and the pixel of the
density data b are the same.
For example, when the density data of a and b are the same, the output
image data for the input image data of the pixel of the density data a and
the pixel of the density data b have characteristics as shown in FIG. 9.
As will be also obviously understood, a transposition ratio of the density
data from the pixel of the density data b to the pixel of the density data
a gradually increases from the density region in which the density data of
each pixel has the half value of the maximum value, namely, from the
halftone density region to the highlight density region. When the density
data is minimum, all of the density data of the pixels of the density data
b is transposed to the pixel of the density data a.
The relation between the input and output of the image data in which the
pixel of the density data a and the pixel of the density data b are added
is linear as shown in FIG. 10. By the above process, the density of the
output data is preserved, the pixel of the density data a and the pixel of
the density data b lie within a low density region and the pixel densities
equivalently decrease, so that the gradation reproducibility in the low
density region is improved. The resolution is preserved.
The above process will now be explained with reference to a flowchart shown
in FIG. 11.
FIG. 11 is a flowchart showing the operation of the density processing unit
of the form of the embodiment.
In step S50, the density data a and b of the adjoining pixels among the
pixels arranged in the raster direction of the line memory 122 is
detected.
In step S52, the sum (a+b) of the density data of the pixels is calculated
on the basis of the density data of the pixels which were detected. When
(a+b) is equal to or smaller than C (YES in step S52), the processing
routine advances to step S54. When (a+b) is larger than C (NO in step
S52), step S56 follows.
When (a+b) is equal to or smaller than C, in step S54, a' and b' are
calculated as new density data in accordance with the equations (1) and
are set to the density data of the pixels.
When (a+b) is larger than C, the detected density data a and b is set to
the density data.
In step S58, the density data a' and b' is transferred to the light
scanning unit 101 as density data of the image data.
By repeating the above operation, the image data which was sequentially
stored in the line memory 122 is properly converted to a desired density
by the density processing unit 123 and is transferred to the light
scanning unit 101.
By modifying the equations (1), the transposition ratio of the pixel of the
density data b can be also changed as shown by the equations (2).
a'=a+b[1-{(a+b)/C}x]
b'=b{(a+b)/C}x (2)
For example, now assuming that the transposition ratio of the b density
changes in accordance with an xth-order function (x is a positive real
number of 1 or more), as the value of x increases, the transposition ratio
of the density data rises, the gradation can be smoothly expressed in the
highlight region, and the gradation reproducibility is improved. On the
contrary, however, the resolution in the highlight region deteriorates. By
setting the value of x so as to obtain a good balance about such a
reciprocal effect, an image at a high picture quality can be outputted. A
preferred value of x is set to 2 to 3.
As described above, in the density data a and b of the adjoining pixels in
the region from the halftone density region to the highlight density
region, since the pixel of the density data b is transposed to the pixel
of the density data a in accordance with the equations (1) or (2), the
pixel density of the main-scanning in the highlight density region is
close to the half and the gradation reproducibility to the output image
data for the input image data can be improved. As the density region
approaches the image region of high density data, the output image data
can be drawn at a high fidelity by the pixel density of the input image
data, so that the resolution of the output image data can be improved.
Further, since the transposition ratio from the pixel of the density data
b to the pixel of the density data a is gradually changed, in the output
image data, no gradation level difference occurs and the deterioration in
picture quality due to the pseudo outline doesn't occur as well.
In the form of the embodiment, the case using the laser beam printer has
been described. However, the invention can be also applied to an ink jet
printer or the like for recording by an emission of an ink.
The value of x in the equations (2) can be also set from a console panel or
a host computer.
The method shown in the embodiment can be applied to a system constructed
by a plurality of equipment or can be also applied to an apparatus
comprising one piece of equipment. It will be obviously understood that
the invention can be also applied to a case where it is embodied by
supplying a program to a system or an apparatus. In this case, a memory
medium in which a program according to the invention has been stored
constructs the invention. By reading the program from the memory medium to
the system or apparatus, the system or apparatus operates in accordance
with a predetermined method.
As described above, according to the embodiments, the image forming
apparatus and method which can improve the gradation reproducibility while
preferably preserving the resolution of the recording image data can be
provided.
Further another embodiment of the invention will now be described. In this
embodiment, there is disclosed an image processing method whereby even in
an apparatus in which an image forming density of the image forming
apparatus can be visually resolved in least distance of distinct vision,
an image can be formed so that a linear density of a formed image cannot
be visually resolved in least distance of distinct vision.
Although a construction of the image forming apparatus of the embodiment is
similar to that shown in FIG. 7, it is now assumed that the image forming
apparatus which is used in the embodiment can express 256 (=8 bits)
gradations every pixel and that the image data has a resolution of 400
d.p.i. A resolution of the main-scanning direction of the image forming
apparatus is set to 400 d.p.i.
In the density processing unit 123, in the case where the density data of
two adjoining pixels in the raster direction (main-scanning direction) is
set to a and b and the image density is equal to or less than the halftone
density, namely, when the image density is smaller than C/2 [C denotes the
maximum density of an image to be formed by the image forming apparatus]
(C/2>a +b), the density data is transposed in accordance with the
following arithmetic operations, thereby newly converting to the pixel
having the density data of a' and b'.
All of the decimal portions of the values which are calculated by the
arithmetic operations are omitted.
The odd trains of the data train in the sub-scanning direction are set to
d=(a+b)/2
a'=2d{1-d/(C/2)}
b=2d.sup.2 /((C/2)
The even trains of the data train in the sub-scanning direction are set to
a'=2d.sup.2 /(C/2)
b'=2d(1-d/(C/2)}
By setting as mentioned above, the dot arrangement in the low density
region becomes a zigzag lattice arrangement and lines of a resolution
which is equal to 283 l.p.i. (lines per inch) at an angle of 45.degree.
for the main-scanning direction are formed. Since those lines have a
density of 250 d.p.i. or more as will be explained hereinlater, the
density level is set to a level such that the dot structure cannot be
confirmed by the human eyes in least distance of distinct vision.
By executing the above process, the output result at each gradation of the
pixel of the density data a and the pixel of the density data b becomes as
shown in FIG. 12.
FIG. 12 shows a dot pattern which is obtained by processing the image data
of the density gradation which gradually increases from the top to the
bottom, namely, from the upper position to the lower position in the
sub-scanning direction.
At a low density, dots are formed like a lattice of an angle of 45.degree.
and have a construction of 45.degree. and 283 l.p.i.
A limit which can be visually resolved by the human eyes in least distance
of distinct vision (about 30 to 40 cm) is about 250 l.p.i. and the dot
construction cannot be seen by the eyes.
In the electrophotography system, in case of individually forming two small
dots, an unstable latent image state occurs; however, by collecting the
dots from two pixels to one pixel at a low density as mentioned above, the
pixel is formed by one larger dot, so that a reproducing state of dots
becomes stable and an effect of reduction of image noises is obtained.
That is, since the dot reproducing state becomes stable, a variation in
size of each dot is eliminated. In case of forming a uniform image of a
uniform halftone density "125" when the maximum density level assumes, for
example, "255", so long as the sizes of dots are inherently uniform, a
smooth image which hardly has what is called, a noise feeling is obtained.
However, there is a problem such that if the sizes of dots are not
uniform, an image with a noise feeling is formed.
On the other hand, according to the method of the embodiment, since the
dots are collected from two pixels to one pixel at a low density, the
stable dots can be formed as mentioned above and an effect of reduction of
the image noises can be obtained.
Further, by previously setting the mean value of two pixels to d=(a+b)/2,
in case of the image density data obtained by scanning a printed matter,
there is a difference of a large value between the density data a and b.
Further, there is obtained an effect to prevent the occurrence of a
situation such that when the period is almost a multiple of two pixels,
there is a fear of occurrence of a deviation in a' and b' which are
finally calculated, so that the correct density cannot be reconstructed.
The above processes will be described with reference to a flowchart of FIG.
14.
In step S50, the density data a and b of the adjoining pixels [the
odd-number designated pixel and the even-number designated pixel] among
the pixels arranged in the raster direction in the image data of a
plurality of lines stored in the line memory 122 is detected.
In step S51, on the basis of the density data of the pixels which was
detected, the sum (a+b) of the density data of the pixels is calculated.
When (a+b) is equal to or smaller than C/2, the processing routine
advances to step S52. When (a+b) is larger than C/2, step S56 follows.
When (a+b) is equal to or smaller than C/2, d is calculated in step S52.
When the sub-scanning is performed for the odd pixel in step S53, the
processing routine advances to step S54. If NO in step S53, namely, the
sub-scanning is performed for the even pixel, step S57 follows.
In steps S54 and S57, a' and b' are calculated by the equations shown above
and are set to the data of the pixels.
When (a+b) is larger than C/2, the density data a and b detected in step
S56 is set to the data of the pixels.
In step S55, the density data a' and b' is transferred to the light
scanning unit 101 as density data of the images.
A check is made to see if the processing of the image data stored in the
line memory 122 has been finished (S59). If NO, the processing routine is
returned to step S50 and the foregoing processing steps S50 to S55 are
repeated. When the processing is finished, a check is made to see if the
data of the next line exists (S61). If NO, the processing routine is
finished. When the data of the next line exists, the data of the next line
is inputted (S63).
It is also possible to construct to change the transposition ratio of the
density data by calculating equations different from the foregoing
equations.
In the embodiment, the transposition of the image data for concentration of
the dots has been performed by the calculating equations shown in steps
S54 and S57 in FIG. 14. According to the calculating equations, however, a
degree of concentration of the dots doesn't Linearly change but
non-linearly changes in accordance with the level of the image signal.
Therefore, the dots can be efficiently concentrated and a large effect is
obtained.
Further, as shown in an embodiment, which will be explained hereinlater,
when a color image is formed, particularly, with respect to the black
component, different from the other color components, there is an effect
such that by increasing the degree of concentration of the dots, the
picture quality is improved.
FIG. 13 shows image density converting characteristics in the case where
the processing steps S51 to S54 and S57 shown in FIG. 14 were executed. A
solid line shows the characteristics of a when the sub-scanning is
performed to the odd pixel, that is, line number of the sub-scanning
direction is odd and a broken line shows the characteristics of b. When
the sub-scanning is performed to the even pixel, that is, line number of
the sub-scanning direction is even, the solid line shows b and the broken
line shows a.
[Other embodiments]
The embodiment relates to the case where the invention is applied to a full
color image forming apparatus of the electrophotography system having a
plurality of drums.
In the full color image forming apparatus of the electrophotography system,
processing steps until an image is formed will now be described.
FIG. 15 is a cross sectional view showing a construction of the full color
image forming apparatus of the electrophotography system of the
embodiment.
The apparatus comprises four stations to form images of four colors of
magenta, cyan, yellow, and black. Photosensitive drums 701a to 701d are
uniformly charged by primary charging units 702a to 702d. Laser beams
emitted from semiconductor lasers (not shown) driven by image signals of
the respective colors are scanned and exposed onto the photosensitive
drums 701a to 701d by a polygon mirror 17, thereby forming latent images.
The latent images are developed by developing units 703a to 703d, so that
toner images are formed on the photosensitive drums 701a to 701d.
A recording material 706 put on a recording material tray 60 is picked up
by a pickup roller 713a and conveyed into the apparatus by a registration
roller 713b. The toner images formed on the drums 701a to 701d are
transferred onto the recording material 706 which is conveyed by a
transfer belt 708. The recording material is subsequently conveyed.
Registration timings of a plurality of color images on the recording
material 706 are matched and images are sequentially multiple transferred
by transfer charging units 704a to 704d. The transfer sheet 708 is
separated by a separation charging unit 14 and by a curvature of a
transfer sheet holding roller 710 and is fixed onto the recording material
by a fixing roller 71 and a pressurizing roller 72. The recording material
is subsequently ejected to the outside of the image forming apparatus.
A fixing step will now be described in detail.
The fixing roller 71 is formed by coating a silicon rubber and a fluorine
rubber onto the surface of a metallic pipe. The pressurizing roller 72 is
formed by coating a silicon rubber onto the surface of a metal roller.
Halogen heaters 75 and 76 are controlled by a thermistor 79 attached to
the surface of the pressurizing roller 72 and by a temperature control
circuit (not shown), thereby controlling the surface temperature to a
predetermined value suitable for fixing.
A silicon oil in an oil pan is pumped and moved to an oil coating roller 77
through a pumping roller 78. By sequence controlling the contact and
removal between the oil coating roller 77 and the fixing roller 71 and by
an oil control blade 80, a predetermined amount of oil is held on the
fixing roller 71.
Each of cleaning devices 73 and 74 uses a belt-like cleaning web member and
can always perform the cleaning by a fresh surface by the feeding and
take-up of the web.
In the full color image formation as shown here, the registration of each
color is an important item. It is well known that even if the registration
of one color is deviated, the picture quality largely deteriorates.
Particularly, in the case where a gray image having a large area of a
certain degree at small ratios of cyan, magenta, yellow, and black as
image forming colors is formed, a color variation easily occurs.
To solve such a problem, according to the embodiment, the phase of a
pattern period of the dot formation is changed by the image forming color.
FIG. 16 shows an example of formation of a specific dot construction
pattern.
Black circles indicate dots of magenta and black and the dot pattern is
formed by the same algorithm as that of the embodiment shown in FIG. 14.
Mesh circles indicate dots of cyan and yellow and the phase of the dot
pattern is inverted from the phase of the black circle and the dot pattern
is formed. To form such a pattern, when embodying the procedure shown in
the flowchart of FIG. 14, it is sufficient to execute the processes of
FIG. 14 for the image data of magenta and black and to execute the
processes in which the processing steps S54 and S57 in FIG. 14 are
exchanged for the image data of cyan and yellow.
By alternately arranging the coordinate positions of the dots in dependence
on the colors, an allowance degree of the deviation of the registration
within a range of the size of the lattice is widened. On the other hand,
when four colors are overlaid at the same coordinates, a saturation
deteriorates. Namely, since the black dot is overlaid to the dots of
magenta, cyan, and yellow, color information to be reproduced by the color
dots drops and the saturation deteriorates.
However, according to the dot construction pattern shown in FIG. 15, the
black dot fundamentally is not overlaid to the dots of magenta and cyan.
Or, a probability such that the black dot is overlaid is small. Therefore,
at least the drop of the color information of magenta and cyan can be
prevented.
Even if the relation between the color and the phase at the dot forming
position is changed to another combination as shown in, for example, FIG.
15, a similar effect is also obtained.
Effects of the embodiment will now be described with reference to FIGS. 16
and 17.
FIG. 17 is a diagram showing the relation between a toner grain size and an
RMS granularity.
An axis of abscissa of FIG. 17 shows the number of lines of a screen
(l.p.i.) and an axis of ordinate indicates the RMS granularity.
Toners in which the average toner grain sizes are equal to 12, 8, and 5
.mu.m are used and their RMS granularities are plotted while changing the
number of lines of the screen.
The RMS granularity is a parameter which is often used in case of measuring
a roughness, namely, image noises in a silver-salt photograph or the like.
Samples of a uniform pattern of the image density D=0.3 are used. The
scanning densities are measured by an aperture of a size of 100
.mu.m.times.100 .mu.and the RMS granularity is calculated by obtaining the
standard deviation of the density data at about 1000 measuring points.
Therefore, it will be understood that as a numerical value of the
granularity is large, a noisy image having a rough feeling is formed.
According to a subjective evaluation, when the granularity has a value
larger than 0.02, the image is evaluated so that it has a picture quality
which cannot be accepted. To set the granularity to a value smaller than
0.02, when extrapolating from the graph data, it is preferable that the
average toner grain size is smaller than 10 .mu.m in the case where the
number of lines of the screen is equal to 200 l.p.i.
FIG. 18 shows visual sense MTF characteristics in least distance of
distinct vision (about 35 cm). An axis of abscissa denotes the resolution
and an axis of ordinate indicates the MTF.
Referring to FIG. 18, it will be understood that in case of observing by
the eyes in least distance of distinct vision, it is impossible to resolve
at 11 lines/mm, namely, about 250 l.p.i. By increasing the number of lines
of the screen to a value larger than 250 l.p.i., the density level is set
to a level at which the dot structure is not recognized by the observer.
In the embodiment of the invention showing the dot forming pattern in FIG.
12 mentioned above, since an image is formed at an angle of 45.degree. and
a resolution of 283 l.p.i., it is possible to prevent that the dot
structure is recognized by the observer.
As shown in FIG. 17, as a tendency of the toner grain size, as the toner
grain size is small, it is advantageous with respect to a roughness. Even
if the number of lines is increased, there is a tendency such that the
roughness is improved.
Explanation will now be made on the basis of the data verified by the
commercially available apparatus which has already been designed and by
model apparatuses made on an experimental basis by the inventors who
applied the method disclosed in the embodiment.
The types shown by A, B, and C in FIG. 19 relate to the commercially
available apparatuses. Laser beam sizes and toner grain sizes
corresponding to the types are set to the values as shown in the diagram.
According to the commercially available types, the image processing shown
in the present specification and drawings is not adopted.
On the other hand, according to the apparatus of the F type of the present
invention, the laser beam size is equal to 45 .mu.m.times.65 .mu.m and the
toner grain size lies within a range from 5 to 6 .mu.m. Further, the image
processing method shown in the specification is adopted.
According to the embodiment, the toner of the grain size of 5 .mu.m is
adopted from the above characteristics, the image signal of 400 l.p.i. is
used, the image processing to concentrate the dots is performed, an image
is formed at an angle of 45.degree. and a resolution of 283 l.p.i. Thus,
the image noises are reduced as much as possible. The image in which the
dot structure cannot be recognized under the ordinary observing conditions
could be formed.
Although the image was formed under the above conditions according to the
embodiment, it will be obviously understood that there is a larger effect
by adopting smaller toner in future.
According to the embodiment as described above, there are provided the
scanning means for scanning a digital image signal necessary to form a
color image and forming an image, comparing means for comparing the
scanned digital image signal with a predetermined image signal value, and
modulating means for modulating the signal in accordance with the image
signal for a group of pixels divided to a predetermined size and
expressing an image dark/light state, wherein in the system in which as a
result of the comparison by the comparing means, when the image signal is
smaller than the predetermined signal value, the signal value is modulated
by the modulating means in the pixels divided into the predetermined size,
the size of the pixel group divided into the predetermined size is set to
a size which cannot be visually resolved in least distance of distinct
vision, there are advantages such that the gradation reproducibility in
the low density region is improved, the density information preservation
is improved, and the image noises are reduced.
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